This invention relates to a printed wiring board on which various electronic components are mounted having on one side thereof lands for forming external electrodes on and an electronic device using this printed wiring board.
Semiconductor devices called BGAs (ball grid arrays) are typical known electronic devices of this kind.
As shown in FIG. 1, a BGA is a device made by mounting a semiconductor device 2 on one side (the upper side) of a printed wiring board 1 of about 1 mm in thickness made of an organic material or an inorganic material and disposing projecting external electrodes 3 in a predetermined array, for example in the form of a matrix, on the other side (the lower side) of the printed wiring board 1. The semiconductor device 2, which is an active component, is connected by for example gold bonding wires 4 to a wiring pattern on the printed wiring board 1 and encapsulated in this state together with the bonding wires 4 by a molded resin 5.
These external electrodes 3, as shown enlarged in FIG. 2A, are each formed on a circular land 6 made of a conducting material such as copper foil, and a pattern-protecting film 7 made of solder resist or the like is formed around the periphery of this land 6, exposing the land 6. The external electrodes 3, which are connecting electrodes for connecting the printed wiring board 1 to a mother board or the like, are formed after the semiconductor device 2 is encapsulated with the molded resin 5 so that they project from the underside of the printed wiring board 1 by heating and thereby melting solder grains or solder paste or the like supplied to the lands 6.
However, in this printed wiring board 1 of the related art, the pattern-protecting film 7 is formed on the peripheral portions of the lands 6 as well as around them, and when the external electrodes 3 are formed on the lands 6 the external electrodes 3 and the pattern-protecting film 7 come into contact with each other. Because of this there has been the problem that when the device is mounted on a mother board or is temperature cycle tested, thermal stresses arise in the base portions of the external electrodes 3 due to a difference in thermal expansion coefficient between the external electrodes 3 and the pattern-protecting film 7 and cracks (microcracks) 3a form in these base portions of the external electrodes 3 as shown in FIG. 2B.
This has resulted in the semiconductor device having fatal flaws such as that crack formation causes external electrodes 3 to come off the printed wiring board 1 and make electrical connection of the printed wiring board 1 to the mother board impossible or the insides of cracks oxidize and the electrical resistance increases, adversely influencing the device characteristics.